1. Field of the Invention
The present invention generally relates to a Universal Serial Bus (USB), and more particularly to a USB 3.0 transaction translator (U3TT) and a micro-frame synchronization method.
2. Description of Related Art
Universal Serial Bus (USB) is a specification of peripheral devices to establish communication between the devices and a host. USB 3.0 was released in 2008, and a USB 3.0 cable is shown in FIG. 1A. In addition to a signal pair D+/D− for USB 2.0 data path, two signal pairs SSTX+/SSTX− and SSRX+/SSRX− are further provided for USB 3.0 SuperSpeed data path.
When a USB 2.0 device is coupled to a USB 3.0 host, communication is performed over the non-SuperSpeed data path D+/D−. The applicant of the present invention discloses a USB 3.0 transaction translator (U3TT) in China Patent Application No. 201010129143.4, the disclosure of which is hereby incorporated by reference. The U3TT 10 as shown in FIG. 1B is coupled between the USB 3.0 host 12 and the USB 2.0 device 14, and is utilized to upgrade the USB 2.0 device to act as a quasi-USB 3.0 device such that data transfer speed can be improved.
USB provides four information transfer types: control transfer, bulk transfer, interrupt transfer and isochronous transfer, among which the isochronous transfer type reserves sufficient bandwidth with guaranteed transfer speed and is commonly used in audio or video data transfer. However, as the isochronous transfer type will not correct errors or retry packets, it cannot guarantee the correctness of transferred data.
As the conventional U3TT stays idle most of the time and the bandwidth usage of the USB 2.0 bus is low when performing IN isochronous transaction, the applicant of the present invention thus discloses an improved U3TT in a co-pending U.S. patent application Ser. No. 12/959,261 entitled USB TRANSACTION TRANSLATOR AND AN ISOCHRONOUS-IN TRANSACTION METHOD. In the improved U3TT, two buffers are used to pre-fetch data to improve the bandwidth usage of the USB 2.0 bus. However, as USB 3.0 micro-frame time (which is 125 μs) at the host side is not synchronized with USB 2.0 micro-frame time, some problems occur.
FIG. 2 shows micro-frame timing diagrams at the host side and device side respectively. At the host side, the host sends isochronous timestamp packet (ITP) every micro-frame to announce its micro-frame time. The host may read data from the buffers, e.g., buffer 1 or buffer 2, at any time during the micro-frame time. At the device side, U3TT sends a start-of-frame (SOF) packet to announce its micro-frame time. The U3TT may controllably perform data pre-fetch during half of the micro-frame time such as the first half of the micro-frame time. As shown in FIG. 2, the host and the device may probably access the same buffer (e.g., the buffer 1) at the same time (e.g., the drawn overlapped period), therefore resulting in access conflict. In this example, the micro-frame at the device side may conflict with the preceding micro-frame at the host side.
Moreover, as the host and the U3TT use different clock sources respectively, a period difference may ordinarily exist between them. The period difference may accumulate to be a substantive difference, therefore resulting in a synchronization problem.
In order to overcome the problems mentioned above, a need has arisen to propose an improved transaction translator and a micro-frame synchronization method to alleviate the synchronization problems.